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are available. No information on population trends are available. In most areas where their ranges overlap Risso’s dolphins are more common than other closely related species, such as pilot whales, false killer whales, melon-headed whales, or pygmy killer whales, although less common than the smaller delphinids. Some evidence of population division exists both between and within ocean basins, based on morphological, genetic, and distribution data, although population boundaries have not been clearly delineated. Seasonal north–south shifts in density have been suggested off the west coast of North America.

III. Ecology Risso’s dolphins are thought to feed almost entirely on squid

(both neritic and oceanic species), and limited behavioral research suggests that they feed primarily at night. Diet may vary by age and sex ( Cockroft et al ., 1993 ). No evidence of predation by either killer whales ( O. orca ) or large sharks is available, although both likely prey on Risso’s dolphins at least occasionally. Mass strandings of this spe-cies are very rare. The range of Risso’s dolphins seems to be limited by water temperature, with animals most common in waters between 15°C and 20°C and rarely found in waters below 10°C.

IV. Behavior and Physiology Risso’s dolphins are relatively gregarious in nature, typically

traveling in groups of 10–50 individuals, with the largest group observed estimated to contain over 4000 individuals. Stable groups of adults have been documented within larger aggregations. Based on the age structure of a school killed in a drive fi shery in Japan, it has been suggested that mature male Risso’s dolphins move among groups. Risso’s dolphins frequently travel with other cetaceans. Off southern California they have been documented to “ bow ride ” on and apparently harass gray whales ( Eschrichtius robustus ), and are often seen “ surfi ng ” in swells. Aggressive behavior directed toward short-fi nned pilot whales ( Globicephala macrorhynchus ) has been observed. No studies on diving behavior have been undertaken.

V. Life History Life history information for this species is relatively limited

( Amano and Miyazaki, 2004 ). Gestation has been estimated at 13–14 months and calving interval at 2.4 years. There appears to be a peak in calving seasonality during the winter months in the eastern Pacifi c and in the summer/fall months in the western Pacifi c. Age at sexual maturity is thought to be 8–10 years for females and 10–12 years for males. The oldest Risso’s dolphin estimated by examining growth layer groups in the teeth was 34.5 years old.

IV. Interactions with Humans Interactions with humans are diverse. Although they occasionally

bow-ride on vessels, in most cases Risso’s seem indifferent to vessels or actively avoid them. Risso’s dolphins have been recorded steal-ing bait from longlines in a number of areas and have been killed as bycatch, as well as being deliberately killed as a result of such inter-actions. Risso’s dolphins are also killed accidentally in gillnet and seine-net fi shing around the world, and have been recorded ingesting plastic and with high levels of contaminants in tissues. Small numbers of Risso’s dolphins have been killed in small-scale whaling operations around the world, and off Sri Lanka and possibly Japan these takes may seriously jeopardize the local population. Risso’s dolphins have been held in aquaria in both Japan and the United States, although

they are relatively uncommon in captivity compared to other species of cetaceans. A number of hybrids with bottlenose dolphins ( Tursiops truncatus ) have been documented in Japanese aquaria.

See Also the Following Article Delphinids, Overview

References Amano , M. , and Miyazaki , N. ( 2004 ). Composition of a school of Risso’s

dolphins, Grampus griseus . Mar. Mamm. Sci. 20 , 152 – 160 . Baird , R. W. , and Stacey , P. J. ( 1991 ). Status of the Risso’s dolphin,

Grampus griseus , in Canada . Can. Field-Nat. 105 , 233 – 242 . Baumgartner , M. F. ( 1997 ). The distribution of Risso’s dolphin ( Grampus

griseus ) with respect to the physiography of the northern Gulf of Mexico . Mar. Mamm. Sci. 13 , 614 – 638 .

Cockroft , V. G. , Haschick , S. L. , and Klages , N. T. W. ( 1993 ). The diet of Risso’s dolphin, Grampus griseus (Cuvier, 1812), from the east coast of South Africa . Z. Säugetierkd. 58 , 286 – 293 .

Gaspari , S. , Airoldi , S. , and Hoelzel , A. R. ( 2007 ). Risso’s dolphin (Grampus griseus ) in UK waters are differentiated from a population in the Mediterranean Sea and genetically less diverse . Cons. Gen. 8 , 727 – 732 .

Kruse , S. , Caldwell , D. K. , and Caldwell , M. C. ( 1999 ). Risso’s dol-phin Grampus griseus (G. Cuvier, 1812) . In “ Handbook of Marine Mammals ” ( S. Ridgway , ed. ) , Vol. 6 , pp. 183 – 212 . Academic Press , New York .

Leatherwood , S. , Perrin , W. F. , Kirby , V. L. , Hubbs , C. L. , and Dahlheim , M. ( 1980 ). Distribution and movements of Risso’s dol-phin, Grampus griseus , in the eastern North Pacifi c . Fish. Bull. US 77 , 951 – 963 .

Shane , S. H. ( 1994 ). Occurrence and habitat use of marine mammals at Santa Catalina Island, California from 1983–91 . Bull. South. Calif. Acad. Sci. 93 , 13 – 29 .

Shane , S. H. ( 1995 a). Behavior patterns of pilot whales and Risso’s dolphins off Santa Catalina Island, California . Aquat. Mamm. 21 , 195 – 197 .

Shane , S. H. ( 1995 b). Relationship between pilot whales and Risso’s dol-phins at Santa Catalina Island, California, USA . Mar. Ecol. Prog. Ser . 123 , 5 – 11 .

Hartman, K. L., Visser, F. and Hendricks, A. J. E. (2008). Social structure of Risso’s dolphins (Grampus griseus) at the Azores: A stratifi ed com-munity based on highly associated units. Can. J. Zool. 86, 294–306.

River Dolphins RANDALL R. REEVES AND ANTHONY R. MARTIN

Most people are surprised to learn that some species of dolphins, and even one porpoise population, live either entirely or partly in freshwater rivers and lakes. Three

species have adapted so thoroughly to freshwater habitats, over mil-lions of years, that they now look very different from their marine-dwelling ancestors. A fourth species, the franciscana, looks similar to those three, is most closely related to one of them, and likely once lived in rivers as well, but it is now found only in coastal marine waters. These four dolphin species, collectively (and ambiguously) known as river dolphins, exhibit some extreme characteristics in their morphology and sensory systems. Until quite recently, their similarities persuaded taxonomists that they were closely related, but genetic evidence shows that they have been separated for millions of years,

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so the species may already be extinct. If so, its demise was due to confl ict with humans sharing its habitat (see later), and it was the fi rst small cetacean to have been wiped out by man.

The outlier in this group, the franciscana ( Pontoporia blainvillei ), lives in coastal marine waters of eastern South America, including the estuary of the Rio de la Plata (River Plate) between Argentina and Uruguay. The modern river dolphins therefore occur in only two continents, South America and Asia. Most questions regarding their origins and how they evolved remain unresolved. In the case of Inia , for example, one hypothesis is that their ancestors entered the Amazon basin from the Pacifi c Ocean approximately 15 million years ago, whereas another is that they entered from the Atlantic Ocean only 1.8–5 million years ago.

II. Behavior and Ecology Little is known about river dolphin societies, and they probably

vary from species to species. A long-term study of botos in Brazil suggests that they have no long-term affi liations, except in the case of mothers and offspring, and that adult males (which are much larger than females) compete for sexual favors both by fi ghting and by dis-playing objects ( Fig. 2 , inset). Males are often wounded in fi ghts, and are heavily scarred by tooth-rakes, leading to depigmentation of the skin and at least contributing to the bright pink coloration for which the species is well known ( Martin and da Silva, 2006 ). For much of the year, females retreat into the furthest reaches of the fl ooded for-est with their calves, whereas adult males mostly remain on the large rivers. This degree of sexual segregation is unusual in dolphins, and its function is unclear, but it may be related to the needs or protec-tion of the calves ( Martin and da Silva, 2004a ). River dolphins seem not to be highly social, with observed group sizes rarely exceeding 10 or 15 individuals. Yet the densities at which they exist, expressed in terms of individuals per unit area of water surface, sometimes far exceed those of marine cetaceans. For example, botos and tucuxis in portions of the upper Amazon system typically occur in densities of 1–10 individuals/km 2 ( Vidal et al ., 1997 ).

Controversy has surrounded the question of whether river dol-phins, like their marine counterparts, communicate with high-pitched

and that their morphological similarity is due largely to convergent evolution. In other words, separately and progressively they have adapted their form and function to life in a similar habitat. They are also among the most seriously threatened cetaceans because their habitat and resources must be shared with many millions of people.

I . Defi nition and Distribution In Rice’s (1998) evaluation of aquatic mammal systematics ,

he assigned the term “ river dolphin ” to the “ peculiar long-snouted ” dolphins in four single-species genera: Platanista (the South Asian river dolphin, known as bhulan in Pakistan, susu in India, and shushuk in Bangladesh), Lipotes (the Chinese or Yangtze river dol-phin, known as baiji in China), Inia (the Amazon and Orinoco river dolphin, or boto), and Pontoporia (the franciscana, or La Plata dol-phin). Rice placed each genus in a separate family, but Heyning (see River Dolphins, Relationships) recognized two clades: Platanistidae, containing only Platanista , and Iniidae, containing the other three genera. Although the genera Lipotes and Pontoporia are clearly monospecifi c, it had been customary until recently to recognize two species of Platanista —the Indus dolphin ( P. minor ) and the Ganges dolphin ( P. gangetica ). They are now provisionally regarded as sub-species: P. gangetica minor in the Indus drainage of Pakistan and P. g. gangetica in the Ganges, Brahmaputra, Megna, Karnaphuli, and Sangu drainage systems of India, Bangladesh, and Nepal ( Fig. 1 ). There are three separate populations of the boto ( Inia geoffrensis ): the Bolivian subspecies I. g. boliviensis in the Madeira River drain-age above the Teotonio Rapids at Porto Velho, the Amazonian sub-species I. g. geoffrensis distributed throughout the Amazon drainage basin except the upper Madeira system, and the Orinoco subspecies I. g. humboldtiana distributed throughout the Orinoco drainage basin ( Fig. 2 ; see amazon river dolphin ). The Bolivian form dif-fers from the others in some fundamental morphological characters (e.g., number of teeth), and may deserve recognition as a separate species ( Banguera-Hinestroza et al ., 2002 ). The baiji ( Lipotes vexil-lifer ) is, or was, endemic to China’s Yangtze River system. In the past, it also occurred at least seasonally in the two large lakes, Dongting and Poyang, appended to the middle reaches of the Yangtze and in the neighboring Qiantang River. A comprehensive survey of the known current range in 2006 failed to encounter a single baiji, and no reliable sightings had been reported in several years before then,

Figure 1 A boto surfaces in its typical riverine habitat, Mamirauá, Brazil, August 2005. Photo by A. R. Martin and P. Gallego. Figure 2 A male boto waves plant material in the air, an action

that may function as part of a social or sexual display. Mamiaruá, Brazil, March 2006. Photos by A. R. Martin and P. Gallego .

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whistles. Studies of the boto have been complicated by the fact that most of its range is shared with a delphinid ( Sotalia fl uviatilis ) that is known to whistle. The question has not yet been answered unequiv-ocally, but very high-frequency whistles of unusual form have been reported from Ecuador in the apparent absence of Sotalia ( May-Collado and Wartzok, 2007 ), so it now seems likely that botos do use whistles, as do (did) baijis ( Wang and Wang, 2006 ).

The small-scale distribution of river dolphins is far from random. They tend to congregate at particular points in a river, especially at confl uences (where rivers or streams converge), sharp bends, sand-banks, and near the downstream ends of islands. In a study of the dis-tribution of Ganges dolphins in Nepal’s Karnali River, Smith (1993) found the animals primarily in eddy countercurrent systems of the main river channel. Such areas of interrupted fl ow occur when fi ne sand or silt is deposited as a result of stream convergence. It is not entirely clear why the dolphins are attracted to these sites, but it likely has some relation to prey availability and energy saving. As Smith (1993) points out, positions within eddies “ require minimal energy to maintain but are near high-velocity currents where the dolphins can take advantage of passing fi sh. ” Large confl uences may contain tens of dolphins at a given time, but such concentrations appear to be adven-titious rather than formed for social reasons. In other words, nonin-teracting individuals are found in close proximity due to the clumped nature of resources and refugia in the river systems where they are found. Indus dolphins and botos have similar behavior in terms of habitat preferences and the nature of feeding aggregations.

River dolphins have a number of physical characteristics that set them apart from other cetaceans. All have evolved relatively small eyes, probably because vision is of limited value in silt-laden water, but the Indus and the Ganges dolphins have moved so far along this path that their eyes now lack a crystalline lens, rendering them functionally blind. At most, they may be able to perceive gross differences between light and dark. These dolphins usually swim on their side, with one fl ipper (most often the right one) trailing near the river bottom and the body oriented so that the tail end is somewhat higher in the water column than the head. Their head nods constantly as they scan acous-tically for prey and obstacles. Indus and Ganges dolphins remain active day and night. All river dolphins are endowed with a sophisticated bio-sonar system, but those other than the Indus and the Ganges dolphins also have good vision. Other physical adaptations to freshwater envi-ronments, where maneuverability may be important in shallow areas and fl ooded forest, are large fl ippers and fl exible bodies. The river dol-phins of the Indus, Ganges, and Amazon/Orinoco all share these char-acteristics, and at least the boto can even swim backward.

All river dolphins have adapted to living in a highly dynamic envi-ronment. Although much of their habitat is silty, they also occur in areas where the water is clear, as in the upper reaches of the Ganges, or “ black ” (stained by tannic acid), as in many Amazon and Orinoco tributaries. Water levels in the Amazon can vary seasonally by as much as 10–13 m. During the low-water season, the dolphins (and other fauna) are restricted to the deep channels of lakes and rivers, whereas during the fl ood season they can range widely. Amazon dol-phins penetrate into rain forests and venture onto grasslands during the fl oods. Their diet seems diverse, with at least 45 fi sh species from 18 families, in addition to crabs and river turtles, represented in examined stomach contents ( Best, 1984 ). Both schooling and non-schooling fi sh species are eaten. Botos are the only modern cetaceans with a differentiated dentition. The teeth in the front half of the jaw are conical, whereas those in the latter half have a fl ange on the inside portion of the crown, more reminiscent of molars (for crush-ing) than canines or incisors (for biting and holding). Presumably,

this feature is related to the hard-bodied or spiny character of some of their prey (e.g., armored catfi shes, even turtles); large catfi sh are often torn into smaller pieces before being eaten.

III. Threats and Conservation Concerns Any description of the river dolphins must include a section on

their conservation status. They include within their ranks some of the most endangered aquatic mammals ( Reeves et al. , 2000 ; Jefferson and Smith, 2002 ). As mentioned earlier, the baiji is either the most critically endangered cetacean species or it is already extinct ( Turvey et al ., 2007 ). Discovered by Western science as recently as 1918, it was apparently still common and widely distributed along the entire Yangtze River, from near the Three Gorges to Shanghai, when China’s Great Leap Forward began in the autumn of 1958. From that time, baijis were probably hunted to some extent for meat, oil, and leather. Although protected legally since the early 1980s, they continued to die accidentally in fi shing gear, from collisions with powered vessels, and from exposure to underwater blasting during harbor construc-tion, in addition to suffering from the effects of overfi shing, pollution, industrial and vessel noise, and the damming of Yangtze tributaries. Efforts to protect the baiji (as well as the Yangtze population of fi n-less porpoises) have been far from adequate. China’s commitment to industrial and agricultural development of the Yangtze basin makes its interest in preserving the natural environment pale by comparison.

The Indus and the Ganges dolphins are also classifi ed as endan-gered, with the former numbering about a thousand and the latter possibly in the low thousands. Indus dolphins occur today only in the main channel of the river, although historically they also inhabited several large tributaries (Sutlej, Ravi, Chenab, and Jhelum). Their population has been fragmented by irrigation dams, and the subpopu-lations trapped upriver of these dams have progressively gone extinct. Now, only two or three subpopulations of Indus dolphins are large enough to be viable. The Ganges dolphin has also lost large segments of upstream habitat as a result of dam construction, but its generally broader distribution makes it less immediately threatened with extinc-tion. Similar to the baiji, the Indus and the Ganges dolphins have been subjected to incidental capture in fi shing gear, especially gill nets. An additional concern for the Ganges dolphin is that fi shermen in some parts of India and Bangladesh use dolphin oil as an attractant while fi shing for a highly esteemed species of catfi sh. This means that there is a demand for carcasses and a disincentive for releasing live dolphins found in nets. Also, some tribal people in remote reaches of the Ganges and the Brahmaputra basins still hunt dolphins for food. A proposed seismic profi ling survey in the Brahmaputra River in Assam in 2006–2007 brought international attention to the emergent risks to Asian river dolphins from oil and gas development.

Because the Amazon has not yet experienced as much modifi ca-tion or resource extraction as other great rivers, boto populations there appear relatively healthy. The Mamirauá reserve, an area of 11,240 km 2 of fl ooded forest in the central Amazon, had an estimated 13,000 animals in the early years of the twenty-fi rst century ( Martinand da Silva, 2004b ). This density cannot be extrapolated across the species ’ range because Mamirauá is unusually productive and has an extremely high density of botos. Nonetheless, the total range-wide population is likely in the high tens of thousands at least. This rosy picture is unlikely to last, however. More and more hydroelectric dams are fragmenting the boto’s range, riverside human popula-tions are burgeoning, bringing increasing resource extraction, and in recent years large numbers of botos have been killed in Brazil for use as fi sh bait.

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Ultimately, all river cetaceans are threatened by the transforma-tion of their habitat to serve human needs. In addition to impeding the natural movements of dolphins and other aquatic organisms, dams in southern Asia divert water to irrigate farm fi elds and sup-ply homes and businesses in an arid landscape, reducing directly the amount of habitat available to the dolphins. As water becomes an increasingly strategic resource in a warming world with expand-ing human populations, the prospects for freshwater cetaceans are certain to deteriorate even further.

See Also the Following Articles Amazon River Dolphin ■ Baiji ■ Endangered Species and Populations■ Finless Porpoise ■ Franciscana ■ Irrawaddy Dolphin ■ Susu and Bhulan ■ Tucuxi

References Banguera-Hinestroza , E. , Cárdenas , H. , Ruiz-García , M. , Marmontel , M. ,

Gaitán , E. , Vázquez , R. , and García-Vallejo , F. ( 2002 ). Molecular identifi cation of evolutionarily signifi cant units in the Amazon river dolphin Inia sp. (Cetacea: Iniidae) . J. Hered. 93 , 312 – 322 .

Best , R. C. ( 1984 ). The aquatic mammals and reptiles of the Amazon .In “ The Amazon: Limnology and Landscape Ecology of a Mighty Tropical River and its Basin ” ( H. Sioli , ed. ) , pp. 371 – 412 . Dr W. Junk, Dordrecht , Dordrecht, The Netherlands .

Best , R. C. , and da Silva , V. M. F. ( 1989 ). Amazon river dolphin, boto Inia geoffrensis (de Blainville, 1817) . In “ Handbook of Marine Mammals ” ( S. H. Ridgway , and R. Harrison , eds ) , Vol. 4 , pp. 1 – 23 . Academic Press , London .

Brownell , R. L. , Jr. ( 1989 ). Franciscana Pontporia blainvillei (Gervais and d’Orbigny, 1844) . In “ Handbook of Marine Mammals ” ( S. H. Ridgway , and R. Harrison , eds ) , Vol. 4 , pp. 45 – 67 . Academic Press , London .

Caldwell, M. C., Caldwell, D. K., and Brill, R. L. (1989). Inia geoffren-sis in captivity in the United States. In “ Biology and Conservation of the River Dolphins ” Occasional Papers of the IUCN Species Survival Commission No. 3 (W. F. Perrin, R. L. Brownell, Jr., K. Zhou, and J. Liu, eds), pp. 35–41. IUCN, Gland, Switzerland .

Chen , P. ( 1989 ). Baiji Lipotes vexillifer Miller, 1918 . In “ Handbook of Marine Mammals ” ( S. H. Ridgway , and R. Harrison , eds ) , Vol. 4 , pp. 25 – 43 . Academic Press , London .

Herald , E. S. , Brownell , R. L. , Jr. , Frye , F. L. , Morris , E. J. , Evans , W. E. , and Scott , A. B. ( 1969 ). Blind river dolphins: First side-swimming cetacean . Science 166 , 1408 – 1410 .

Jefferson, T. A., and Smith, B.D. (eds.) (2002). Facultative freshwater cetaceans of Asia: Their ecology and conservation. Raffl es Bull. Zool. Suppl. 10, 187 pp .

Kasuya , T. ( 1999 ). Finless porpoise Neophocaena phocaenoides (G. Cuvier, 1829) . In “ Handbook of Marine Mammals ” ( S. H. Ridgway , and R. Harrison , eds ) , Vol. 6 , pp. 411 – 442 . Academic Press , San Diego .

Martin , A. R. , and da Silva , V. M. F. ( 2004 a ). River dolphins and fl ooded for-est: Seasonal habitat use and sexual segregation of botos Inia geoffrensisin an extreme cetacean environment . J. Zool. (Lond). 263 , 295 – 305 .

Martin , A. R. , and da Silva , V. M. F. ( 2004 b ). Number, seasonal move-ments and residency characteristics of river dolphins using an Amazonian fl oodplain lake system . Can. J. Zool. 82 , 1307 – 1315 .

Martin , A. R. , and da Silva , V. M. F. ( 2006 ). Sexual dimorphism and body scarring in the boto (Amazon river dolphin) Inia geoffrensis . Mar. Mamm. Sci. 22 ( 1 ) , 25 – 33 .

May-Collado , L. J. , and Wartzok , D. ( 2007 ). The freshwater dolphin Iniageoffrensis geoffrensis produces high frequency whistles . J. Acoust. Soc. Am. 121 ( 2 ) , 1203 – 1212 .

Perrin, W. F., Brownell, R. L., Jr., Zhou, K., and Liu, J. (eds.) (1989). “ Biology and Conservation of the River Dolphins. ” Occasional Papers

of the IUCN Species Survival Commission No.3. IUCN–The World Conservation Union, Gland, Switzerland .

Reeves , R. R. , and Brownell , R. L. , Jr. ( 1989 ). Susu Platanista gangetica(Roxburgh, 1801); and Platanista minor Owen, 1853 . In “ Handbook of Marine Mammals ” ( S. H. Ridgway , and R. Harrison , eds ) , Vol. 4 , pp. 69 – 99 . Academic Press , London .

Reeves, R. R., Smith, B. D., and Kasuya, T. (eds.) (2000). “ Biology and Conservation of Freshwater Cetaceans in Asia. ” Occasional Papers of the IUCN Species Survival Commission No. 23. IUCN–The World Conservation Union, Gland, Switzerland.

Rice , D. W. ( 1998 ). “ Marine Mammals of the World: Systematics and Distribution . ” Society for Marine Mammalogy , Lawrence, KS , Special Publication No. 3 .

Smith , B. D. ( 1993 ). 1990 status and conservation of the Ganges river dolphin Platanista gangetica in the Karnali River, Nepal . Biol.Conserv. 66 , 159 – 169 .

Smith , B. D. , Thant , U. H. , Lwin , J. M. , and Shaw , C. D. ( 1997 ). Investigation of cetaceans in the Ayeyarwady River and northern coastal waters of Myanmar . Asian Mar. Biol. 14 , 173 – 194 .

Turvey, S. T., Pitman, R. L., Taylor, B. L., Barlow, J., Akamatsu, T., Barrett, L. A., Zhao, X., Reeves, R. R., Stewart, B. S., Wang, K., Wei, Z., Zhang, X., Pusser, L. T., Richlen, M., Brandon, J. R., and Wang, D. (2007). First human-caused extinction of a cetacean species? Biol. Letters 3, 537–540.

Vidal , O. , Barlow , J. , Hurtado , L. A. , Torre , J. , Cendón , P. , and Ojeda , Z. ( 1997 ). Distribution and abundance of the Amazon river dolphin (Inia geoffrensis ) and the tucuxi ( Sotalia fl uviatilis ) in the upper Amazon River . Mar. Mamm. Sci. 12 , 427 – 445 .

Wang , K. , and Wang , D. ( 2006 ). Estimated detection distance of a bai-ji’s (Chinese river dolphin, Lipotes vexillifer ) whistles using a passive acoustic survey method . J. Acoust. Soc. Am. 120 , 1361 – 1365 .

Zhou , K. , and Zhang , X. ( 1991 ). “ Baiji: The Yangtze River Dolphin and Other Endangered Animals of China . ” Stone Wall Press , Washington, DC .

River Dolphins, Evolutionary History and Affi nities

CHRISTIAN DE MUIZON

The term “ river dolphins ” or Platanistoids has been traditionally used to include the recent odontocetes that live in freshwater and are not members of the other clades of odontocetes: del-

phinoids, ziphioids, and physeteroids. Their affi nities to other groups of odontocetes were unresolved, mainly because they have many ple-siomorphic characters (e.g., Slijper, 1936 , Simpson, 1945 ). There are four genera of living “ river dolphins ” ( Platanista , Lipotes , Inia , and Pontoporia ). Other (partly) freshwater odontocetes include Orcaella(Irrawadi River) and Sotalia (Amazon River) are not included in the Platanistoidea because they are clearly related to the marine dol-phins, Delphinidae. Although it was previously assumed that platan-istoids were monophyletic, this is almost certainly not the case, and some of their included taxa have been regarded as closely related to several groups of fossil odontocetes: e.g., the Squalodontidae, the Eurhinodelphinidae, the “ Acrodelphinidae. ” There is now consen-sus that Platanistoidea is para- or polyphyletic ( Muizon, 1984, 1987, 1988, 1991, 1994 ; Heyning, 1989 ; Fordyce, 1994 ; Messenger and McGuire, 1998 ; Fig. 1 ).

The genus Platanista appears to be an early diverging group of odontocetes, Platanistoidea, and the three other genera ( Lipotes ,